The present invention relates to a structural body (fine structural body) employing an aluminum member having on a surface an anodized film with micropores present, and to a method of producing the same.
In technical fields of thin films, thin wires, dots, and the like of metals or semiconductors, an electrically, optically, or chemically specific phenomenon is known to occur due to confinement of motions of free electrons in the metals or semiconductors each having a size smaller than a certain characteristic length. Such a phenomenon is referred to as “quantum-mechanical size effect (quantum size effect)”. Research and development of functional materials applying such a specific phenomenon have been currently conducted actively. To be specific, a material having a fine structure of less than several hundred nm is referred to as a “fine structural body” or a “nanostructural body”, and is regarded as one of the targets for material development.
An example of a method of producing such a fine structural body includes a method of directly producing a nanostructural body through a semiconductor processing technique including a fine pattern formation technique such as photolithography, electron beam exposure, or X-ray exposure.
Of those, researches on a method of producing a nanostructural body having an ordered fine structure have received attention, and have been conducted extensively.
An example of a method of forming a self-regulated, ordered structure involves subjecting aluminum to anodizing treatment in an electrolyte, to thereby obtain an anodized alumina film (anodized film). The anodized film is known to have ordered fine pores (micropores) each having a pore size of about several nm to about several hundred nm. A perfectly ordered arrangement may be obtained utilizing self-ordering property of the anodized film. The perfectly ordered arrangement theoretically has cells of hexagonal prisms each having a base of an equilateral hexagon formed around a micropore as a center. Lines connecting adjacent micropores are known to form equilateral triangles.
For example, H. Masuda et al., Jpn. J. Appl. Phys., Vol. 37 (1998), pp. L1340-1342, Part 2, No. 11A, 1 Nov. 1998 (FIG. 2) describes an anodized film having variation in pore size of micropores within 3%. Further, “Surface Finishing Handbook”, edited by The Surface Finishing Society of Japan (1998), The Nikkan Kogyo Shimbun, Ltd., pp. 490-553 describes natural formation of pores on an anodized film in the course of oxidation. Further, Hideki Masuda, “Highly ordered metal nanohole arrays based on anodized alumina”, Solid State Physics, 1996, Vol. 31, No. 5, pp. 493-499 proposes formation of an Au dot array on an Si substrate using a porous oxide film as a mask.
The greatest characteristic of the anodized film as a material is that micropores are arranged parallel at substantially equal intervals in a direction substantially vertical with respect to a surface of a substrate to take a honeycomb structure. An additional characteristic thereof is that a pore diameter, a distance between pores, and a pore depth of the micropores can be controlled relatively freely unlike those of other materials (see Hideki Masuda, “Highly ordered metal nanohole arrays based on anodized alumina” (supra).
Application examples of the anodized film include various devices such as a nanodevice, a magnetic device, and a phosphor. For example, JP 2000-31462 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”) describes application examples thereof including: a magnetic device having micropores filled with Co or Ni as a magnetic metal; a magnetic device having micropores filled with ZnO as a phosphor; and a biosensor having micropores filled with an enzyme/antibody.
Further, in a field of biosensing, JP 2003-268592 A describes an example of a structure having micropores of an anodized film filled with a metal as a sample holder for Raman spectroscopic analysis.
Raman scattering refers to scattering of incident light (photon) caused by collision of the incident light with a particle, which induces inelastic collision with a particle and change in energy. Raman scattering is used as a technique for spectroscopic analysis, but the scattering light for measurement must have an enhanced intensity for improvement of sensitivity and precision of analysis.
A surface-enhanced resonance Raman scattering (SERRS) phenomenon is known as a phenomenon for enhancing Raman scattered light. The phenomenon further enhances scattering of light by molecules of certain species adsorbed on a surface of a metal electrode, a sol, a crystal, a deposited film, a semiconductor, or the like compared to scattering in a solution. A significant enhancing effect of 1011 to 1014 times may be observed on gold or silver, in particular. A mechanism for causing the SERRS phenomenon is not yet clarified, but surface plasmon resonance presumably has an effect thereon. JP 2003-268592 A also aims at utilizing the principle of plasmon resonance as means for enhancing the Raman scattering intensity.
Plasmon resonance refers to a phenomenon causing interaction of a plasmon wave, which is a localized electron density wave, with an electromagnetic wave (resonance excitation) to form a resonance state when a surface of a noble metal such as gold or silver is irradiated with light and the surface of the metal is brought into an excited state. Particularly, surface plasmon resonance (SPR) refers to a phenomenon causing collective vibrations of free electrons when a surface of a metal is irradiated with light and the free electrons on the metal surface are brought into excited states. Further, the surface plasmon resonance causes surface plasmon waves to generate strong electric field.
An electric field is enhanced by several orders (108 to 1010 times, for example) in a region in the vicinity of a surface where plasmon resonance takes place, more specifically, in a region within about 200 nm from the surface, and significant enhancement in various optical effects are observed. For example, when light enters a prism having a thin film of gold or the like deposited at an angle of a critical angle or more, a change in dielectric constant of a surface of the thin film can be detected with high sensitivity as a change in intensity of reflected light due to the surface plasmon resonance phenomenon.
To be specific, use of an SPR apparatus applying the surface plasmon resonance phenomenon allows measurement of a reaction amount or bonding amount between biomolecules, or kinetic analysis without labeling and at real-time. The SPR apparatus is applied to researches on immune response, signal transduction, or interaction between various substances such as proteins and nucleic acids. Recently, a paper describing analysis of a trace amount of dioxins using an SPR apparatus has also been reported (see I. Karube et al., Analytica Chimica Acta, 2001, Vol. 434, No. 2, pp. 223-230).
Various methods for enhancing plasmon resonance have been studied, and a technique of localizing plasmon by forming a metal into isolated particles, not into a thin film is known. For example, JP 2003-268592 A describes a technique of localizing plasmon by providing metal particles in pores of an ordered anodized film.
A research report describes that when localized plasmon resonance of metal particles is utilized, the metal particles locating close to one another enhances an intensity of an electric field at a gap between the metal particles, thereby realizing a state where plasmon resonance occurs more easily (see Takayuki Okamoto, “Researches on interaction of metal nanoparticles and on biosensors”, on-line, URL: http://www.plasmon.jp/reports/okamoto.pdf, searched on Nov. 27, 2003). That is, locating metal particles close to one another becomes a critical requirement in a device employing localized plasmon resonance. For example, it is important that the metal particles be located close to one another within a distance of 200 nm without being in contact with each other.